The present invention relates to a sheet-fed rotary printing press for effecting printings to both obverse and reverse sides of a cut sheet.
Conventional two-sided printing machines have been classified into two types. In the first type of printing machine, a cut sheet is conveyed between an obverse side printing unit and a reverse side printing unit, so that dual side printings can be simultaneously achieved. More specifically, U.S. Pat. No. 4,471,695 commonly assigned to Ryobi Ltd. (corresponding to Japanese Patent Application Kokai No. 58-25961) discloses the two-sided printing machine in which two blanket cylinders are in rolling contact with each other, and one of the blanket cylinders of the obverse side printing unit serves as an impression cylinder for reverse side printing, and the other blanket cylinder of the reverse side printing unit serves as an impression cylinder for the obverse side printing. Further, copending U.S. patent application Ser. No. 225,642 filed on Jul. 27, 1988 (corresponding to Japanese Patent Application Kokai No. 62-102532) discloses a double-sided printing machine in which a sheet-feed cylinder is in rolling contact with an impression cylinder of the obverse side printing unit, and the sheet-feed cylinder serves as an impression cylinder for the reverse side printing.
In the second type of printing machine, tandem printing is achieved in such a manner that obverse side printing is first carried out, and then the sheet is turned over for performing the reverse side printing as described in U.S. Pat. No. 4,621,576 assigned to Heidelberger Druckmaschinen AG (corresponding to Japanese Patent Application Kokai No. 60-255434). In the second type, multi-color printing is of convertible type for effecting multi-color printing on the specific side of the printing sheet, and a sheet transfer cylinder is provided with a sheet turnover mechanism.
According to the first type of printing machine in which the opponent blanket cylinder functions as the impression cylinder and vice versa, sufficient printing pressure cannot be applied to the printing sheet, since the blanket cylinder is formed of an elastic material such as a rubber. When the sheet pressedly passes through the blanket cylinders, these cylinders may be elastically deformed, to thereby degrade printing pressure, to thus render an output image which is degraded. In this connection, this type of printing machine has been exclusively used for both-sided printing with a single color such as character printing and line-image printing. Further, due to unwanted elastic deformations, the confronting blanket cylinders cannot exclude two sheets superposedly supplied thereto. As a result, the two sheets are overlappingly subjected to printings.
In case of the printing machine in which the sheet-feed cylinder serves as the impression cylinder for the reverse side printing according to another first type printing machine, it would be almost impossible to provide a sufficient space for installing a sheet infeeding mechanism such as a swing type sheet feeding mechanism, since the reverse side printing unit must be installed at a positon below the sheet-feed cylinder, the reverse side printing unit including a blanket cylinder, a plate cylinder and inking and dampening units. With such spacial limitation, a simple paper feeding mechanism such as an infeed roller the only available, option and therefore, the resultant printing machine is not capable of high speed, high accuracy printing, and sheet size may be restricted in the printing machine.
According to the second type of printing machine, the sheet turnover mechanism is of complicated construction and complicated operation may be required. Accordingly, high dimensional accuracy is required in the overall apparatus. Further, for the turnover operation to the sheet, sheet is regripped, and therefore, the gripped sheet position may be changed to thereby lower print positional registration. As a result, high quality output image may not be obtainable. Furthermore, in the sheet turnover operation, since the sheet end portion is regripped, cutting of the sheet stack requires high accuracy. Therefore, available sheet size may undergo restriction.